Waves - Reflection 01

This GCSE Physics quiz on waves asks questions about reflection. Waves transfer energy from one location to another and travel in straight lines. But that isn't always the case. When a wave hits a surface that it cannot penetrate, and if the surface does not absorb all the energy the wave carries, then it will bounce off the surface and travel in a new direction. This is reflection and you will be most familiar with it from your studies of light waves but any wave can be reflected.

When light waves strike a plane or curved mirror, they bounce off the shiny surface. The key point from the law of reflection is that the angle of reflection equals the angle of incidence. So if you want to turn a wave through 90°, the wave must hit the surface at 45°.

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To measure the angles, you need a reference point, this is called the normal. The normal is a construction line that is drawn at 90° to the surface of the mirror (or whatever the reflecting surface is) at the point of incidence of the ray of light being studied. Angles are measured from the normal and not the surface of the mirror. Mirrors are used in telescopes (unsurprisingly called reflecting telescopes) for astronomical use, examples of these telescopes are Newtonian reflectors and the more complicated Cassegrain reflectors.

There are two types of reflection - regular and irregular (sometimes called diffuse). Regular reflection produces a clear image such as the image seen in a mirror. It is produced by reflection off smooth, polished surfaces. This type of image is a virtual image. It is as far behind the mirror as the object is in front, the right way up and same size as the object but it is laterally inverted. Laterally inverted means that it is swapped round horizontally. If you look at your reflection in a mirror and raise your right hand, your reflection will appear to raise its left hand. Writing appears to be reversed, which is why some ambulances have the word 'ambulance' written in 'mirror writing' on their bonnet.

Irregular reflection occurs when a wave is reflected by a rough surface. This produces no image because the individual rays of the wave coming into your eye from the object are all reflected through different angles and are coming from different parts of the reflecting surface. Although some surfaces can feel quite smooth, on a microscopic scale they are quite rough. All you see is the reflecting surface itself, no image.

Waves can become trapped inside objects because of reflection. This is a special type of reflection called total internal reflection. It happens where a wave enters a long thin object through one of the ends. The usual example of this at GCSE is in glass fibres. The fibres have a small diameter compared with their length so any light entering through the end of the fibre will hit the outside surface at a low angle and be reflected instead of escaping from the fibre. Total internal reflection is very efficient and light can be transmitted through glass fibres over long distances.

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  1. Which of the following statements are true?
    This is why reflection is so predictable
  2. What is the normal?
    It is a construction line, in other words, it is a line drawn to help with calculations of reflection (and refraction)
  3. A ray of light is shone onto a plane mirror with an angle of incidence of 32 degrees. What is the angle between the reflected ray and the normal?
    The angle of incidence is always measured between the ray and the normal line
  4. What are properties of images produced by plane mirrors?
    Pepper's ghost relies on these properties. If you haven't heard of this look it up in a physics book or online
  5. Which of the following devices uses the principle of reflection?
    Reflection of light is very useful indeed!
  6. What is an incoming ray of light called?
    In the study of waves, incident always indicates something to do with the incoming wave
  7. What is the law of reflection?
    A wave will always reflect off a surface at the same angle at which it hit
  8. Which type of surface would provide the best reflection?
    A polished smooth surface would be even better!
  9. A ray of light reflects from a plane mirror with an angle of reflection of 64 degrees. What was its angle of incidence?
    Remember the law of reflection
  10. What will all waves do when they are incident, at a certain angle, on surfaces that do not allow the waves to pass through?
    They bounce off the surface in a predictable way

Author: Martin Moore

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